Target相关

Target相关

通常我们是调用requestBuilder#into(ImageView)，接下来看看，ImageView是如果转化为Target的。

// RequestBuilder.java
public ViewTarget<ImageView, TranscodeType> into(@NonNull ImageView view) {
    // ... 省略scaleType相关代码

    // 可以看到是通过GlideContext构建一个ViewTarget，内部是使用imageViewTargetFactory来构建的
    return into(
        glideContext.buildImageViewTarget(view, transcodeClass),
        /*targetListener=*/ null,
        requestOptions,
        Executors.mainThreadExecutor());
}

// GlideContext.java

// 通过下方代码分析，可以知道是在Glide的构造函数中传进来的
private final ImageViewTargetFactory imageViewTargetFactory;

public GlideContext(
      ...
      @NonNull ImageViewTargetFactory imageViewTargetFactory,
      ...) {
    super(context.getApplicationContext());
    ...
    this.imageViewTargetFactory = imageViewTargetFactory;
    ...
}


public <X> ViewTarget<ImageView, X> buildImageViewTarget(ImageView imageView, Class<X> transcodeClass) {
    return imageViewTargetFactory.buildTarget(imageView, transcodeClass);
}

// Glide.java
Glide(...){

    ImageViewTargetFactory imageViewTargetFactory = new ImageViewTargetFactory();
    glideContext = new GlideContext(
            ...
            imageViewTargetFactory,
            ...);
}
    

// ImageViewTargetFactory.java
public class ImageViewTargetFactory {
  
  // 通过判断不同的类型，实例化不同的ViewTarget
  public <Z> ViewTarget<ImageView, Z> buildTarget(
      @NonNull ImageView view, @NonNull Class<Z> clazz) {
    if (Bitmap.class.equals(clazz)) {
      return (ViewTarget<ImageView, Z>) new BitmapImageViewTarget(view);
    } else if (Drawable.class.isAssignableFrom(clazz)) {
      return (ViewTarget<ImageView, Z>) new DrawableImageViewTarget(view);
    } else {
      throw new IllegalArgumentException(
          "Unhandled class: " + clazz + ", try .as*(Class).transcode(ResourceTranscoder)");
    }
  }
}

通常我们是通过一个url去加载图片的，由上面RequestManger#load(string)代码可以知道，RequestBuilder的transcodeClass是Drawable.class，所以最后得到的target是DrawableImageViewTarget。

从Glide源码分析二——Request相关中的SingleRequest#begin()可知，如果没有设置overrideWidth/overrideHeight，会通过viewTarget#getSize(SizeReadyCallback)获取大小，获取大小后在回调SizeReadyCallback#onSizeReady()通知SingleRequest加载图片。

public abstract class ViewTarget<T extends View, Z> extends BaseTarget<Z> {
        
    protected final T view;
    private final SizeDeterminer sizeDeterminer;
    
    public ViewTarget(@NonNull T view) {
        this.view = Preconditions.checkNotNull(view);
        sizeDeterminer = new SizeDeterminer(view);
    }
  
    public void getSize(@NonNull SizeReadyCallback cb) {
        sizeDeterminer.getSize(cb);
    }  
    
    
    static final class SizeDeterminer {
    
        private final View view;
        private final List<SizeReadyCallback> cbs = new ArrayList<>();
        private SizeDeterminerLayoutListener layoutListener;
        
        SizeDeterminer(@NonNull View view) {
            this.view = view;
        }
        
        void getSize(@NonNull SizeReadyCallback cb) {
          int currentWidth = getTargetWidth();
          int currentHeight = getTargetHeight();
          // 如果view已经可以获取大小了，则回调SizeReadyCallback#onSizeReady(w,h)通知加载图片
          if (isViewStateAndSizeValid(currentWidth, currentHeight)) {
            cb.onSizeReady(currentWidth, currentHeight);
            return;
          }

          if (!cbs.contains(cb)) {
            cbs.add(cb);
          }
          if (layoutListener == null) {
            
            // 通过添加PreDrawListener来获取大小，获取大小后会回调SizeReadyCallback#onSizeReady(w,h)通知加载图片
            ViewTreeObserver observer = view.getViewTreeObserver();
            layoutListener = new SizeDeterminerLayoutListener(this);
            observer.addOnPreDrawListener(layoutListener);
          }
        }
        
        // 获取view的高度
        private int getTargetHeight() {
          int verticalPadding = view.getPaddingTop() + view.getPaddingBottom();
          LayoutParams layoutParams = view.getLayoutParams();
          int layoutParamSize = layoutParams != null ? layoutParams.height : PENDING_SIZE;
          return getTargetDimen(view.getHeight(), layoutParamSize, verticalPadding);
        }
        
        private int getTargetDimen(int viewSize, int paramSize, int paddingSize) {
          
          int adjustedParamSize = paramSize - paddingSize;
          
          // layoutParam中的Size可用就用paramSize
          if (adjustedParamSize > 0) {
            return adjustedParamSize;
          }
          
          // 如果view申请了绘制（例如调用了requestLayout），则返回0（无效尺寸，后面会去测量）
          if (waitForLayout && view.isLayoutRequested()) {
            return PENDING_SIZE;
          }

          // 如果view已经有了宽高，则返回view的宽高
          int adjustedViewSize = viewSize - paddingSize;
          if (adjustedViewSize > 0) {
            return adjustedViewSize;
          }

          // 如果view没有申请绘制，且尺寸是wrap_content，则返回屏幕宽高的最大值
          if (!view.isLayoutRequested() && paramSize == LayoutParams.WRAP_CONTENT) {
            
            return getMaxDisplayLength(view.getContext());
          }

          // 不满足上述条件则返回无效尺寸
          return PENDING_SIZE;
        }
        
        
        void checkCurrentDimens() {
          if (cbs.isEmpty()) {
            return;
          }

          int currentWidth = getTargetWidth();
          int currentHeight = getTargetHeight();
          if (!isViewStateAndSizeValid(currentWidth, currentHeight)) {
            return;
          }

          // 获取到view的大小后，通过SizeReadyCallback#onSizeReady(w,h)通知加载图片
          notifyCbs(currentWidth, currentHeight);
          
          // 回调完后清除之前创建的PreDrawListener和添加的SizeReadyCallback
          clearCallbacksAndListener();
        }
        
        
        private static final class SizeDeterminerLayoutListener
        implements ViewTreeObserver.OnPreDrawListener {
          private final WeakReference<SizeDeterminer> sizeDeterminerRef;

          SizeDeterminerLayoutListener(@NonNull SizeDeterminer sizeDeterminer) {
            sizeDeterminerRef = new WeakReference<>(sizeDeterminer);
          }

          @Override
          public boolean onPreDraw() {
            // 这时可以正常获取到大小了，通过SizeDeterminer去获取大小
            SizeDeterminer sizeDeterminer = sizeDeterminerRef.get();
            if (sizeDeterminer != null) {
              sizeDeterminer.checkCurrentDimens();
            }
            return true;
          }
        }
        
    }

}

ViewTarget通过SizeDeterminer获取大小，当view已经有大小或可以通过LayoutPrams获取大小时，直接回调；否则通过添加PreDrawListener来获取大小，之后再回调。

public abstract class ImageViewTarget<Z> extends ViewTarget<ImageView, Z>
    implements Transition.ViewAdapter {
    
  // 其他onLoadXxx(...)都差不多
  @Override
  public void onLoadStarted(@Nullable Drawable placeholder) {
    super.onLoadStarted(placeholder);
    setResourceInternal(null);
    setDrawable(placeholder);
  }
  
  // 当资源获取成功时会回调此方法
  @Override
  public void onResourceReady(@NonNull Z resource, @Nullable Transition<? super Z> transition) {
    if (transition == null || !transition.transition(resource, this)) {
      setResourceInternal(resource);
    } else {
      maybeUpdateAnimatable(resource);
    }
  }
  
  private void setResourceInternal(@Nullable Z resource) {
    // 通知子类加载资源，例如DrawableImageViewTarget就是直接调用ImageView#setImageDrawable(resource)
    setResource(resource);
    // 如果资源是动画的话，则开启动画
    maybeUpdateAnimatable(resource);
  }
  
  @Override
  public void setDrawable(Drawable drawable) {
    view.setImageDrawable(drawable);
  }
  
  private void maybeUpdateAnimatable(@Nullable Z resource) {
    if (resource instanceof Animatable) {
      animatable = (Animatable) resource;
      animatable.start();
    } else {
      animatable = null;
    }
  }
  
  protected abstract void setResource(@Nullable Z resource);
    
}